1. Field of the Invention
This invention relates in general to the field of stray-light detection and diagnosis in optical systems and, in particular, to a novel device and method for stray-light detection and diagnosis by illumination of the aperture of an optical system with a subaperture light beam to facilitate tracing stray light to its source.
2. Discussion of Related Art
For an optical imaging system to produce a good image, the system must produce an acceptable wavefront quality over the field-of-view and maintain acceptable control of stray-light. This is generally true for any imaging system, regardless of the operating wavelength band. Stray light can obscure faint signals, decrease the signal-to-noise ratio, reduce image contrast, create inaccurate radiometric and photometric results and, in high energy laser systems, destroy optical elements and detectors. Stray light becomes a critical issue for optical systems that must look at a low-level signal on a bright background, such as in the case of long-wave infrared imaging systems.
Stray-light in a system can arise from multiple sources including design defects such as inadequate baffles, manufacturing problems such as damage to a coating or baffle edge, incorrect assembly and foreign object debris. Determining the cause of stray-light can be a very challenging problem. Much can be accomplished through analysis of a design via simulation, but simulation is not proof of a system's performance. Furthermore, simulation does not account for a system that does not conform to the design, which occurs for example due to component damage, incorrect assembly, or foreign object debris.
Historically, stray-light problems are resolved through design and build methods that depend heavily upon analysis or are ad hoc. In either case, the detection, diagnosis and mitigation of a stray light problem is very time consuming. Stray-light testing in these scenarios uses full aperture illumination, such as in the stray-light test station of Breault Research Organization, described in G. L. Peterson, “Stray light test station for measuring point source transmission and thermal background of visible and infrared sensors,” Proc. of SPIE Vol. 7069, 70690M (2008). In this test station full-aperture illumination is used to detect the presence of stray-light in an optical system from illumination at a particular direction, e.g., field and azimuth angles, relative to the system under test.
Determination of the cause of the stray light produced by full-aperture illumination requires extensive analysis of a minimal data set comprised of the knowledge that stray light occurs when light comes from a specific direction. This data is used, in combination with analysis, to hypothesize where the stray light originates; however, proof requires corrective action to the system (disassembly, modification and reassembly) and repetition of the test to demonstrate the stray-light is no longer present.
What is needed is a more efficient means to determine the specific cause of stray light in a system for system design evaluation, system production and testing of systems in the field.